The present invention relates to the field of wind turbine generators, and more particularly to the field of horizontal-axis wind turbines. One of the principal problems involved in designing horizontal axis wind turbines is wind shear, which is the variation of wind velocity with height above ground level. Wind velocities tend to increase with altitude due to aerodynamic surface drag and the viscosity of air. As a result, turbine blades at the top of the rotation experience higher wind velocities than blades at the bottom of the rotation. If not compensated for in the design of the wind turbine, this vertical wind velocity gradient will both degrade the performance and efficiency of the wind turbine and subject it to damaging stresses.
In addition to wind shear due to natural differences in wind velocity with altitude, wind shear can also be induced by improper alignment of the main shaft axis, i.e., not facing the axis at the optimal angle with respect to the wind direction. Most often, improper alignment results from changes in wind direction. If there is no wind shear, the rotor axis (the axis around which the blades are rotating) should face directly into the wind so that all blades will experience the same wind speed. If however, the main shaft axis is aligned obliquely to the wind in one direction, blades at the top of the rotation will move into the wind, and blades at the bottom of the rotation will move with the wind. This will cause blades at the top of the rotation to experience a greater effective wind speed than blades at the bottom. Conversely, if the orientation of the main shaft is oblique to the wind in the opposite direction, blades at the bottom of the rotation will experience a greater effective wind speed than those at the top.
Of vital importance in the design of wind turbine generators is operation of the turbine blades at the optimum tip speed ratio to extract as much power as possible out of the wind. Tip speed ratio is defined as the speed at the tips of the turbine blades divided by the speed of the wind. For example, if the wind is blowing at 20 mph and the blade tips are rotating at 100 mph, then the tip speed ratio is 5. If however, there is a wind velocity difference of 10 mph between the lowest and highest blade positions, the tip speed ratio will vary from 4 to about 7, thereby diverging from the optimum design point with consequent loss of efficiency. Variations in tip speed ratio due to wind shear also cause changes in the angle of attack of the turbine blades, which depends on the speed of the blades relative to the wind speed. The effect is to increase the angle of attack at the top of the blade's path and decrease it at the bottom. In the above example, the angle of attack will be increased by almost 3 degrees at the top and decreased by almost 3 degrees at the bottom. This can result in stall at the top and reduced lift power at the bottom.
The lift generated by turbine blades during rotation is applied both in the direction of rotation and in a backward direction. Forces applied in the direction of rotation are also designated as in-plane forces and forces applied in a backward direction are also designated as out-of-plant forces. These backward forces are usually substantially greater than the forces applied in the direction of rotation. Because of this, wind shear will cause more backward force to be applied to blades experiencing the greater effective wind speed. The stress produced by this unbalance in backward forces is augmented by the concomitant changes in the angle of attack of the blades. This cyclical stress on the blades and bearings can cause excessive wear, maintenance problems, and shorten the useful life of the wind turbine generator.
The prior art in this field has responded to the problems presented by wind shear through the use of a “teeter pin” that is part of the hub. A teeter pin provides for an additional degree of freedom by enabling the turbine rotor to pivot back-and-forth like a playground seesaw. The addition of the teeter pin causes a backward force to be converted to backward torque. This back-and-forth rotation results in a balancing of the torque on the blades around the teeter axis because blades experiencing the higher wind velocity move with the wind and blades experiencing the lower wind velocity move into the wind. Such teeter pins are useful as applied to two-bladed wind turbines, as they allow the upper blade to tilt backward while the lower blade tilts forward. Thus, the teetering motion of a two-bladed wind turbine tends to equalize the effective wind speeds for both blades, thereby maintaining a constant tip speed ratio. Teetering also tends to equalize the backward torque on both blades, thereby reducing shear stresses.
The limited seesaw pivoting enabled by teeter pins is, however, inadequate to compensate for wind shear in turbines having three or more blades. This is because teetering is limited to one blade moving forward and the other moving backward in an equal and opposite manner. The concept of the present invention is to provide a ball-and-socket hub that enables back-and-forth rotation of blades for turbines having two or more blades. The center of the ball-and-socket hub is defined as the origin of x, y and z axes. As shown in FIG. 2A, the y-axis is parallel to the yaw axis, through which the nacelle rotates about the tower to change the orientation of the rotor with respect to the wind. The z-axis is aligned with the main shaft axis and generally with the direction of the wind, and the x-axis is aligned horizontally and roughly parallel to the ground.
The hub has a series of dynamic rotational couplers between the ball and socket that rotationally couples the ball and socket around the z-axis so that rotational torque generated by the turbine blades is transferred from the socket to the ball. In addition the dynamic rotational couplers enable the socket to move back-and-forth with respect to the ball in response to the back-and-forth rotation of the blades. This back-and-forth rotation provides an additional degree of freedom.
The back-and-forth rotation of the blades exhibits a cyclic pattern that is dependent upon the gradient in wind speed. For example, again referring to FIG. 2A, if blades are rotating clockwise around the rotor axis (as viewed from the front), and there is a vertical gradient in wind velocity, blades will start to rotate backward around the x-axis due to higher wind speed starting at the 180° position and continue rotating backward a until reaching the maximum backward position at 0°. Then the blades will begin to rotate forward due to lower wind velocity until the maximum forward position is reached at 180°. The term “wind shear axis” is used to describe an axis which is perpendicular to the gradient in wind shear, such that the maximum forward and maximum backward rotation positions occur along the wind shear axis. In the above example, the x-axis is the wind shear axis. This rotation of the blades around the wind shear axis causes a rotation of the hub socket around the hub axis, which is perpendicular to the wind shear axis. In the above example, the hub axis is the y-axis. The hub axis is not a physical axis such as the teetering axis, but rather a virtual axis created by rotation of the socket around the ball. The hub axis is perpendicular to the rotor axis and to the wind shear axis, and can be any axis in the x-y plane that intersects the origin of the x, y, and z axes. The rotation of the hub socket around the hub axis is not a separate degree of freedom, but rather a necessary outcome imposed by the geometry of the ball-and-socket. The extent of rotation around the hub axis is determined by the extent of back-and-forth rotation of the blades.
FIGS. 2B and 2C show the relationship between the back-and-forth rotation of the blades around the wind shear axis 233 and the resulting rotation of the rotor axis 222 around the tub axis 235.
As with teetering, the back-and-forth rotation of the blades occurs in order to balance torque on the blades. This balance occurs across the wind shear axis, e.g., the x-axis. Each blade applies a torque around the wind shear axis based upon the backward force multiplied by the distance from the wind shear axis. When the back-and-forth rotations are such that the torque contributions above and below the wind shear axis are balanced, the back-and-forth rotations will reach a steady state. At this time, the rotation of the hub socket around the hub axis will stop and the angle where it stops becomes the optimal hub angle (the angle between the rotor axis and the main shaft axis). In cases where there is no wind shear, the rotor axis will point directly into the wind. Otherwise, the rotation of the hub socket around the hub axis will orient the rotor axis obliquely into the wind and create rotations that largely offset the wind shear. As changes in wind direction occur, the blade torque above and below the wind shear axis will become unbalanced and cause the extent of the back-and-forth rotations to change. This change will in turn cause the hub socket to rotate around the hub axis until a new optimal hub angle is achieved.
Since the hub axis is perpendicular to the rotor axis, rotation of the hub socket around the hub axis will cause a change in orientation of the rotor axis. This change in orientation of the rotor axis, however, does not affect the orientation of the main shaft axis which remains fixed along the z-axis. An essential consequence of this change in orientation of the rotor axis is that the blades retain rotational symmetry and thus balance the forces of the rotor with respect to both the rotor axis and the main shaft axis. The central feature of the present invention, the ball-and-socket hub, will now be described in further detail.